1. Field of the Invention
The present invention is directed generally to a circuit arrangement for a free-running blocking-oscillator type switched power pack and, in particular, to a switched power pack for generating an output voltage by the clocked application of a dc-voltage to a winding of a transformer through the use of a control means for pulse-width-modulated driving of an electrical switch element. The control means bases the pulse width modulated drive on a primary current signal received at a first input post, a zero axis crossing signal received at a second input post, and a difference signal acquired from a comparitor having a first input connected to a reference voltage and a second input connected to a control voltage.
2. Description of the Related Art
A circuit arrangement is disclosed in German Published Application DE No. 33 12 209 A1 which includes a control means that, among other things, has two control signals supplied to it for pulse width modulated driving of an electrical switch element which is, for example, a thyrister or a power MOS field effect transistor. First, the control means is provided with information regarding the zero axis crossing of the switching pulses to recognize the existing frequency and the existing pulse duty ratio of the switch element. Second, this control means is supplied with a control voltage dependent upon the output voltage of the switched power pack. The control voltage is supplied through a resistor to the inverting input of a controlled gain amplifier in the form of an operational amplifier in the control means at whose non-inverting input a first reference voltage is applied. The control means is thus informed of the pulse start and of the pulse width for driving the switch element.
It has been shown in the known circuit arrangement that the operating frequency of the switched power pack rises up to its natural frequency, which is typically in the range of 150 kHz. through 200 kHz. with increasing unloading of the secondary side of the transformer. This rise in frequency leads to overheating of the switch element which can lead to the destruction thereof. To avoid having to provide a basic load at the secondary side of the switched power pack and, thus, increase dissipation of power in the standby mode, it is proposed in the known circuit arrangement to provide a capacitive switching matrix array between the input posts which receive the zero axis crossing detector signal and the control voltage. The zero axis crossing detector signal is thereby superimposed on the voltage value of the control voltage. As a result of the superimposition of the signals, a number of pulse starts for driving the switch element can be suppressed. As a consequence thereof, frequency division of the characteristic resonance of the switched power pack is achieved so that a lower operating frequency results.
However, it has turned out that at the reduced operating frequency the output voltage of the switched power pack exhibits an undesirable increase of about 10 to 20%. This is particularly disadvantageous since the output voltage is already slightly increased in the standby operating mode. A further increase of the output voltage is therefore undesirable.